I pretty much rate games on pure enjoyment factor. My 10's are all games that give me bliss when playing. However, I can't help but wonder if convenience is a factor, conscious or unconscious, in my ratings or those of others. Various factors that make game inconvenient are (my ratings shown in parentheses):

Setup Time:War of the Ring Collector's Edition (9) is an awesome game, but lugging that beast to the table, setting it up, packing it back up, and lugging it back can easily be more than 30 minutes. For me, it's usually worth it for 3-4 hours of Middle Earth joy. But there are times when I just don't want to be bothered. Likewise with Arkham Horror (7). There are a lot of bits and cards to setup, and some choices to make. It's even the case with Pandemic (9) and Pandemic: On the Brink (9). Even if playing a multiple-game session, spending 5 minutes setting up the deck (random event cards, mutations, virulent strain) seems cumbersome for a 20-to-30-minute game.

Play Time: I'm still at the point in Paths of Glory (9) where it takes more than 1 hour per turn. This is prohibitive for me since I rarely have the opportunity to play it. Although I take almost exactly 1 hour per game for my turns, Through the Ages: A Story of Civilization (10) is often criticized for taking too long. Is it simply inexperience, or is the game just too complex for some?

Learning the System: This one's difficult because it's a barrier to entry, but should not be a long-term problem. But it can be a big enough barrier that many don't push themselves over it. Advanced Squad Leader (9) is one such game, although most medium+ wargames are well beyond the complexity of the heaviest euros.

Playing Well:Bridge (I don't know how to rate it properly) and Go (10) are the best examples. You could learn Bridge in an evening, and Go in 10 minutes, but spend a year deep in study and still be a beginner. I love a steep learning curve, but many people seem to need to win early and often in order to like a game.